Render on cob
Render on wooden lath
Render on metal lath
Render on blocks
Rendering with a spray gun

The correct specification for any render should consider the nature and
condition of the background, site exposure, time of the year (weather maps
/ rainfall and wind driven rain indices) and type of finish required.

The success of a render depends on ensuring good background preparation
and suction control, the correct choice of a mortar and its application.
Sample panels should always be carried out.
The durability of a render depends on mortars that will adhere to the background,
are able to breathe and resist harsh climatic conditions that can and do
occur even in relatively benign climate zones. A good bond to the substrate
and between all coats is essential to the soundness of the render structure.
Bonding is both physical and mechanical:

-

A physical bond is achieved by controlling the suction correctly,
such that a suction bond develops. The natural surface condition can
also offer a good key.

-

Mechanical bonding is induced by the method of application. Ensuring
good keying between layers, and especially the first coat, by casting/harling
or spraying is by far the most successful method.

To avoid potential de-bonding and cracking each coat should be not be richer
in binder or thicker than the preceding one (thicker base coats are applicable
on thin stipple/scratch coats).

Sands for renders.
In dubbing out, stipple coats and base coats the sands should be well graded,
washed and free of clay/silt (particles below #200 mesh). Use sharp sands
from #6 down to #200, with the bulk of the sand in the #16 down to #100.
Fine sands or monogranular sands (bulk in 1 or 2 grades only) are to be
avoided.
In finishing coats, finer sands, still well graded, can be used for smooth
finishes (avoid overtrowling). Particular attention will have to be paid
to finishing coats with fine sands to avoid high shrinkage due to the high
amount of water that fine sands absorb. The use of a wooden float, energetically
applied in small circular motions, will help. Floating with plastic floats
is not suitable. Sponge floats can be used after the wooden float work is
completed to achieve a particular texture in the finish. Curing will also
be important. Small hairline shrinkage cracks can be healed if treated in
time with a light water mist.

Note: the finer sand particles are the ones mostly responsible for
color and therefore used for color rendition. If the fines denote presence
of clay (particles below #200 mesh) the NHL binder quantity should be reduced
(clays are also binders!). A wet sieving analysis is recommended to check
clay / silt content.

Background Preparation: Check that any movement cracks
are stable and where necessary ensure they are properly tied and if needed,
grouted/pinned/pointed. Careful removal of existing renders will result
in less remedial repairs prior to re-rendering. Removal of failed or inappropriate
existing render or finishes, including many types of paint, may require
the walls to be left to dry out properly before re-rendering and time should
be allowed for this. Ensure all repairs to the background are completed
and that loose pinning stones or defective bricks are repaired or replaced
prior to commencement of any rendering. Partial or complete re-pointing
/ consolidation may be required. Remove all loose and friable materials,
remove and treat all organic growth, use biocides where applicable, ensuring
that they will not affect the mortar.
Newly built walls should be allowed to dry properly, usually 1 month.
This will not take place readily in winter conditions.

Repointing before rendering: if this is necessary
it should be done with a compatible mortar.

Detailing: inspect all details, i.e. copings etc.
Check gutters and down pipes and all forms of roof drainage, ground
drainage and general ground conditions. Make sure all the above
items are functioning properly and where remedial action is required,
ensure it is completed before proceeding with render work.
Rendering should never come into contact with soil. Renders should
be kept clear of the ground or finish at the base of a wall into free
draining gravel.

Dubbing out: on defaced surfaces or in areas with
a large amount of damaged joints it will be necessary to apply a
dubbing out coat to provide a level surface. In most cases this
will be sufficient with mortar, however very deep joints or hollows
should be pinned to reduce the mass of mortar. When a dubbing out
coat is used, let it set sufficiently (8-10 hours) before scraping
it and keying it. Apply the first coat after approx. 2 days (more
if very deep recesses have been filled) and depending on weather
conditions.
Dubbing out should leave a relatively flat surface, keyed as necessary,
on which to render.

Suction control: if needed, apply sufficient water to
reduce excessive suction, especially on bricks and porous stone. Old
bricks often require more water than new ones. On many occasions this
is done the day before, if necessary several times with the last damping
just before application starts. Apply water starting at the top of
the structure. Over saturation of the background will result in loss
of bond. Never render backgrounds that have standing water on the
surface. Always dampen preceding coats before applying next coat.

It should be noted that in the presence of diferent suction levels the
degree of dampening will vary accordingly.

Keying: provide adequate keying between background
and base coat and between each coat. Crisscross patterns are preferred to
combing. Make sure that keying does not cut too deeply. Sometimes joints
in brickwork are raked back (normally 1/2"), this is not necessary
with NHL renders if a stipple coat is applied cast on, harled or sprayed
on.Two coat work
Two coat work is suitable for renders with an overall thickness of approx.
5/8" on surfaces that provide adequate suction and a good key. On surfaces
offering poor suction and keying, it is recommended to use a stipple coat
(1/8"-1/4" thick) applied by casting on, harling or spraying.
The main coat can be applied after sufficient hardening and finished as
required. Alternatively use 3 coat work by applying a finishing coat.
On two coat work the base coat will be the thickest (up to 1/2", more
if applied in 2 passes) and with a binder: sand ratio of 1:1.5 or 1:2. Use
mainly NHL 5 or NHL 3.5.
This can be laid on or preferably cast/sprayed on. Scour back and
key after initial setting.To ensure a flat and uniform surface see "Ensuring a level
surface" under Undercoat in 3 coat work section.

Curing: check for initial shrinkage. If found, dampen
surface lightly with water and tighten back and re-key. Repeated shrinkage
is usually a function of poor quality sands, poor suction control
or rapid drying.

Smooth and light textured finishes: use finer well graded
sands, #14 down to #200 mesh. Add just enough water to obtain required workability.
The more water is added the higher the risk of shrinkage. When the mortar
is firm enough, proceed to float up with a cross-grained wood float. This
is the most important phase of the finishing work and should be done diligently.
Together with good curing and protection it is vital in obtaining a good
finish.

Coarse finishes: use coarser sands if thick (rustic)
granular finishes are required. The thickness of the coat depends
on the final finish required. Some of these finishes, especially the
ones requiring special skills such as cottage, scraped and travertine
effects, could also be done by using the same type of sand as smooth
and light textured (floated) finishes. In these and tooled renderings
(patterned), if initial shrinkage takes place, lightly dampen the
surface and re-float the area during the first day or two. Tooling
is normally applied when the render is 5-7 days old.

Two coat work (contd.)

Dry dashing: throw the chosen aggregate onto
soft mortar and leave exposed. To speed up the work a plasterer
throwing the aggregate can follow the laying on plasterer.

Curing: curing by water mist over 3 to 4 days,
if necessary more than once a day, is essential when weather
conditions would cause quick drying. Always protect the work
from adverse weather conditions.

First coat: has to provide sufficient bonding. Stipple or
spatterdash can be used on all backgrounds, but especially on impervious
and smooth background. Leave these coats rough to provide a key. Use richer
mix (1:1.5 preferably). The normal thickness is between 1/8"-1/4".
On soft or weak background use 1:2 or 2:5. Successive coats must be weaker
than this coat. The thickness of the first coat depends on the nature of
the background and the overall thickness required of the render.

A laid on scratch coat can be used on old bricks or surfaces providing
a good key (greater care is required in application to ensure good
bonding with the background). It will be scoured back with a cross
grained wood float and keyed (crisscross keying pattern preferred)
once initial stiffening has taken place.

Second coat (straightening): to be applied 2 days (or
more, depending on weather conditions) after completion of first coat.
Its strength should be less than the first coat. Thickness will
vary according to the overall thickness required but it is normally between
1/2"-5/8". It must not be over 3/4" thick. If this is required
it should be done in successive coats each not exceeding 3/4". The
thicker the intermediate coats, the longer the waiting time before each
subsequent application.

Ensuring a level surface: to achieve a uniform and level surface
fix vertical timber battens or dab's on the wall at 6-8 feet intervals.
If the wall is uneven use spacers and check that battens are straight with
a plumb level. Fill out to screeds, if necessary in layers. Screed off excess
mortar between battens with a wooden straightedge spanning between the battens.
When battens are taken down, fill in strips with the same mortar.
An alternative is to make running screeds 4" wide at regular intervals.
Scour back and key as usual after initial setting. Check for shrinkage
during the first 2 days and, if necessary, lightly dampen the relevant
area, tighten back and re-key. In case of intermediate coats this
would apply to each coat. Do not apply finishing coat until undercoat
is adequately hardened.

Three coat work (contd.)

Finishing coat and curing: as per 2 coat work.

Protecting NHL mortars and renders
The setting properties of NHL mortars require protection against adverse
weather conditions. Precautions are necessary and, if in doubt, your St.
Astier Distributor will be able to advise further. The following are the
main recommendations:

Mortars made with

Protect from frost, rain, strong wind or direct
sun for minimum

NHL 5

48 hours

NHL 3.5

72 hours

NHL 2

96 hours

Early exposure to rain will cause some moisture absorption in the surface
of a fresh render. If frost occurs, there might be damage. The figures given
above refer, therefore, to a render that has not been subject to water penetration
in its early life.

The preferred form of protection is burlap covers
that, with re-damping, will also contribute to curing the mortar. Burlap
covers are essential to protect against frost. Plastic sheeting is effective
against rain but should be kept clear of fresh work. If too tight it will
generate condensation leading to unsightly staining. It will not protect
against frost. Frost protection should be provided even if frost is not
occurring at the moment of finishing the day's work but is forecast during
the early days of a mortar. Work should not start in frost conditions or
when frost is forecast or with temperatures below 40F. In working with NHL
2 or in rendering with fine finishing coats, this should be 45F. Protection
from the quick drying effects of wind or direct strong sun should be provided
by using shading sheets on scaffolding.

Good working practices

In this document we have already discussed items such as background preparation,
suction control, detailing, keying, protection and curing. A good and durable
result depends mainly on these factors, the correct mortar mix, sand, dosages
and workmanship. One item not to be overlooked is scaffolding.
Where scaffolding is being used make sure that the scaffolding has adequate
clearance from the face of the wall to allow application, avoiding unsightly
lift lines. Scaffolding should project past all areas to be rendered to
allow for protection of the new work against direct rainfall. Generally
scaffolding should be capable of carrying the protective screens necessary
to shade the work and prevent rapid uncontrolled drying and any covers needed
to protect against frost.

This document is a guide only and is not intended to be a
specification. Its purpose is to provide the reader with helpful information
that may assist in determining the correct choice of materials, methods
of application and determine the best working practice. The guidelines
refer to our experience with St. Astier NHL binders and some recommendations
might not be applicable to other products.

Bad preparation of background.Over saturated
background.Too much binder.Too many fines in
sand.Finishing coat to thick.Too much water in the
mix.Rapid drying / lack of protection.
Too much sun or wind during curing.

Check if movement is still
active. (Engineer to
check). If building stable,
repair cracks / areas.

Note: a wide variety of finishes can be achieved by adopting different
binder and sand mixes to satisfy all requirements.

Coat thickness and optional mix ratios are related to exposure and background
conditions and are the responsibility of the designer.

Curing and protection must follow best working practice.

Lime

Mix A
1:1.5

Mix B
1:2

Mix C
1:2.5

Mix D
1:3

Mix E
1:4

NHL 2

1A

1B

1C

1D

1E

NHL 3.5

2A

2B

2C

2D

2E

NHL5

3A

3B

3C

3D

Sands

Type

Particle sizes (Mesh)

SG

Sharp gritty

#4 down to #200

SC

Sharp coarse

#6 down to #200

SM

Sharp medium

#8 down to #200

SF

Sharp fine

#16 down to #200

FS

Fine soft

#20 down to #200

.

This document is a guide only and is not intended
to be a specification. Its purpose is to provide the reader with helpful
information that may assist in determining the correct choice of materials,
methods of application and the best working practice. The guidelines refer
to our experience with St. Astier NHL binders and some recommendations might
not be applicable to other products.